In electrophotography photoconductive materials are used to form a latent electrostatic charge image that is developable with finely divided colouring material, called toner.
The developed image can then be permanently affixed to the photoconductive recording material, e.g. a photoconductive zinc oxide-binder layer, or transferred from the photoconductor layer, e.g. a selenium or selenium alloy layer, onto a receptor material, e.g. plain paper and fixed thereon. In electrophotographic copying and printing systems with toner transfer to a receptor material the photoconductive recording material is reusable. In order to permit rapid multiple printing or copying, a photoconductor layer has to be used that rapidly loses its charge on photo-exposure and also rapidly regains its insulating state after the exposure to receive again a sufficiently high electrostatic charge for a next image formation. The failure of a material to return completely to its relatively insulating state prior to succeeding charging/imaging steps is commonly known in the art as "fatigue".
The fatigue phenomenon has been used as a guide in the selection of commercially useful photoconductive materials, since the fatigue of the photoconductive layer limits the copying rates achievable.
A further important property which determines the suitability of a particular photoconductive material for electrophotographic copying is its photosensitivity, which must be sufficiently high for use in copying apparatuses operating with the fairly low intensity light reflected from the original. Commercial usefulness also requires that the photoconductive layer has a spectral sensitivity that matches the spectral intensity distribution of the light source e.g. a laser or a lamp. This enables, in the case of a white light source, all the colours to be reproduced in balance.
Known photoconductive recording materials exist in different configurations with one or more "active" layers coated on a conducting substrate and include optionally an outermost protective layer. By "active" layer is meant a layer that plays a role in the formation of the electrostatic charge image. Such a layer may be the layer responsible for charge carrier generation, charge carrier transport or both. Such layers may have a homogeneous structure or heterogeneous structure.
Examples of active layers in said photoconductive recording material having a homogeneous structure are layers made of vacuum-deposited photoconductive selenium, doped silicon, selenium alloys and homogeneous photoconducting polymer coatings, e.g. of poly(vinylcarbazole) or polymeric binder(s) molecularly doped with an electron (negative charge carrier) transporting compound or a hole (positive charge carrier) transporting compound such as particular hydrazones, amines and heteroaromatic compounds sensitized by a dissolved dye, so that in said layers both charge carrier generation and charge carrier transport take place.
Examples of active layers in said photoconductive recording material having a heterogeneous structure are layers of one or more photosensitive organic or inorganic charge generating pigment particles dispersed in a polymer binder or polymer binder mixture in the presence optionally of (a) molecularly dispersed charge transport compound(s), so that the recording layer may exhibit only charge carrier generation properties or both charge carrier generation and charge transport properties.
According to an embodiment that may offer photoconductive recording materials with particularly low fatigue a charge generating and charge transporting layer are combined in contiguous relationship. Layers which serve only for the charge transport of charge generated in an adjacent charge generating layer are e.g. plasma-deposited inorganic layers, photoconducting polymer layers, e.g. on the basis of poly(N-vinylcarbazole) or layers made of low molecular weight organic compounds molecularly distributed in a polymer binder or binder mixture.
Useful organic charge carrier generating pigments (CGM's) belong to one of the following classes:
a) perylimides, e.g. C.I. 71 130 (C.I.=Colour Index) described in DBP 2 237 539; PA1 b) polynuclear quinones, e.g. anthanthrones such as C.I. 59 300 described in DBP 2 237 678; PA1 c) quinacridones, e.g. C.I. 46 500 described in DBP 2 237 679; PA1 d) naphthalene 1,4,5,8-tetracarboxylic acid derived pigments including the perinones, e.g. Orange GR, C.I. 71 105 described in DBP 2 239 923; PA1 e) tetrabenzoporphyrins and tetranaphthaloporphyrins, e.g. H.sub.2 -phthalocyanine in X-crystal form (X-H.sub.2 Pc) described in U.S. Pat. No. 3,357,989, metal phthalocyanines, e.g. CuPc C.I. 74 160 described in DBP 2 239 924, indium phthalocyanine described in U.S. Pat. No. 4,713,312 and tetrabenzoporphyrins described in EP 428,214A; and naphthalocyanines having siloxy groups bonded to the central metal silicon described in published EP-A 243,205; PA1 f) indigo- and thioindigo dyes, e.g. Pigment Red 88, C.I. 73 312 described in DBP 2 237 680; PA1 g) benzothioxanthene derivatives as described e.g. in Deutsches Auslegungsschrift (DAS) 2 355 075; PA1 h) perylene 3,4,9,10-tetracarboxylic acid derived pigments including condensation products with o-diamines as described e.g. in DAS 2 314 051; PA1 i) polyazo-pigments including bisazo-, trisazo- and tetrakisazo-pigments, e.g. Chlordiane Blue C.I. 21 180 described in DAS 2 635 887, trisazo-pigments, e.g. as described in U.S. Pat. No. 4,990,421 and bisazo-pigments described in Deutsches Offenlegungsschrift (DOS) 2 919 791, DOS 3 026 653 and DOS 3 032 117; PA1 j) squarylium dyes as described e.g. in DAS 2 401 220; PA1 k) polymethine dyes; PA1 l) dyes containing quinazoline groups, e.g. as described in GB-P 1,416,602 according to the following general formula: ##STR1## in which R and R.sub.1 are either identical or different and denote hydrogen, C.sub.1 -C.sub.4 alkyl, alkoxy, halogen, nitro or hydroxyl or together denote a fused aromatic ring system; PA1 m) triarylmethane dyes; and PA1 n) dyes containing 1,5 diamino-anthraquinone groups. PA1 o) inorganic photoconducting pigments e.g. Se, Se alloys, As.sub.2 Se.sub.3, TiO.sub.2, ZnO, CdS, etc. PA1 a) dicyanomethylene and cyano alkoxycarbonylmethylene condensates with aromatic ketones such as 9-dicyanomethylene-2,4,7-trinitrofluorenone (DTF); 1-dicyanomethylene-indan-1-ones as described in EP 537,808 A with the formula: ##STR2## wherein R.sup.1, R.sup.2, X and Y have the meaning described in said EP 537,808 A; PA1 d) substituted 9-dicyanomethylene fluorene compounds as disclosed in U.S. Pat. No. 4,562,132; PA1 e) 1,1,2-tricyanoethylene derivatives. PA1 i) interfacial mixing between the CGL and the CTL resulting in CGM-doping of the CTL and CTM-doping of the CGL causing charge trapping; PA1 ii) charge trapping in the CGL; PA1 iii) poor charge transport in the CGL; PA1 iv) poor charge transport blocking properties in the absence of a blocking layer. PA1 EPON 1001 PA1 EPON 1002 PA1 EPON 1004 PA1 EPON 1007 PA1 EPON 1009 PA1 DER 331 PA1 DER 667 PA1 DER 668 PA1 DER 669 PA1 ARALDITE GT 6071 PA1 ARALDITE GT 7203 PA1 ARALDITE GT 7097 PA1 ARALDITE GT 6099 PA1 ARALDITE GY 281 from Ciba-Geigy. PA1 ARALDITE MY 721 from Ciba-Geigy. PA1 DEN 431 PA1 DEN 438 PA1 DEN 439 PA1 ARALDITE GY 1180 PA1 ARALDITE EPN 1138 PA1 piperidine PA1 triethylamine PA1 benzyldimethylamine (BDA) PA1 2-dimethylaminomethylphenol (DMAMP) ##STR11## 2,4,6-tris(dimethylaminomethyl)phenol (TDMAMP) ##STR12## PA1 i) aromatic poly NH-group amines and other amines e.g. PA1 iii) cycloaliphatic poly NH-group amines e.g. isophorondiamine derivatives commercially available as EPILINK 420 (tradename) from Akzo, The Netherlands; PA1 iv) heterocyclic poly NH-group amines e.g. 4-aminomethylpiperidine ##STR15## v) aliphatic amines e.g. polyoxypropylene amines commercially available under the tradename JEFFAMINE from Texaco Chemical Company e.g. JEFFAMINE T-403 with the general formula: ##STR16## in which c+d+e is about 5.3 JEFFAMINE D-230 with the general formula: ##STR17## in which f is about 2.6 JEFFAMINE M-300 with the general formula: ##STR18## in which g is about 2. PA1 a) perylimides, e.g. C.I. 71 130 (C.I.=Colour Index) described in DBP 2 237 539, PA1 b) polynuclear quinones, e.g. anthanthrones such as C.I. 59 300 described in DBP 2 237 678, PA1 c) quinacridones, e.g. C.I. 46 500 described in DBP 2,237,679, PA1 d) naphthalene 1,4,5,8-tetracarboxylic acid derived pigments including the perinones, e.g. Orange GR, C.I. 71 105 described in DBP 2 239 923, PA1 e) tetrabenzoporphyrins and tetranaphthaloporphyrins, e.g. H.sub.2 -phthalocyanine in X-crystal form (X-H.sub.2 Pc) described in U.S. Pat. No. 3,357,989, metal oxyphthalocyanines, metal phthalo-cyanines, e.g. CuPc C.I. 74 160 described in DBP 2 239 924, indium phthalocyanine described in U.S. Pat. No. 4,713,312, tetrabenzoporphyrins described in EP 428,214A, silicon naphthalocyanines having siloxy groups bonded to the central silicon as described in EP-A 0243205 and X- and B-morphology H.sub.2 Pc(CN).sub.x, H.sub.2 PC(CH.sub.3).sub.x and H.sub.2 PcCl.sub.x pigments, PA1 f) indigo- and thioindigo dyes, e.g. Pigment Red 88, C.I. 73 312 described in DBP 2 237 680, PA1 g) benzothioxanthene-derivatives as described e.g. in DAS 2,355,075, PA1 h) perylene 3,4,9,10-tetracarboxylic acid derived pigments including condensation products with o-diamines as described e.g. in DAS 2 314 051, PA1 i) polyazo-pigments including bisazo-, trisazo- and tetrakisazo-pigments, e.g. Chlordiane Blue C.I. 21 180 described in DAS 2 635 887, and bisazopigments as described in DOS 2 919 791, DOS 3 026 653 and DOS 3 032 117, PA1 j) squarilium dyes as described e.g. in DAS 2,401,220, PA1 k) polymethine dyes. PA1 l) dyes containing quinazoline groups, e.g. as described in GB-P 1,416,602 according to the following general formula: ##STR25## PA1 (1) overall electrostatically charging, e.g. with corona-device, the photoconductive material containing in a charge generating layer said hardened polymeric structure as a binding agent; PA1 (2) image-wise photo-exposing said layer thereby obtaining a latent electrostatic image, that may be toner-developed.
Preferred non-polymeric materials for negative charge transport are:
compounds with the formula: ##STR3## wherein: A is a spacer linkage selected from the group consisting of an alkylene group including a substituted alkylene group, a bivalent aromatic group including a substituted bivalent aromatic group; S is sulfur, and B is selected from the group consisting of an alkyl group including a substituted alkyl group, and an aryl group including a substituted aryl group as disclosed in U.S. Pat. No. 4,546,059; PA2 4,4'-diaminodiphenylmethane (DDM)-derivatives commercially available as EPICURE 153 from Shell Chemical and ARALDITE HY 830 from Ciba-Geigy; PA2 4,4'-diaminodiphenylsulphone; PA2 1,3,5-tris(4'-aminophenyl)benzene ##STR13## 3,5-diphenylaniline ##STR14## ii) poly NH-group amines wherein aliphatic amino groups are attached to an aromatic backbone e.g.: PA2 meta-xylylene diamine commercially available as EPILINK MX from Akzo, The Netherlands; PA2 phenalkamines on the basis of cashew nut shell liquid commercially available as CARDOLITE NC541 and CARDOLITE NC541 LV from Cardolite Corporation.
and 4-dicyanomethylene 1,1-dioxo-thiopyran-4-one derivatives as disclosed in U.S. Pat. No. 4,514,481 and U.S. Pat. No. 4,968,813, e.g. ##STR4## b) derivatives of malononitrile dimers as described in EP 534,004A; c) nitrated fluorenones such as 2,4,7-trinitrofluorenone and 2,4,5,7-tetranitrofluorenone;
The choice of binder for the charge generating layer (CGL) for a given charge generating pigment (CGM) and a given charge transport layer (CTL) has a strong influence on the electro-optical properties of the photoreceptors. One or more of the following phenomena can have a negative influence on the electro-optical properties of the photoconductive recording material:
Interfacial mixing between the CGL and the CTL can be avoided by using a CGL-binder or binders, which is/are insoluble in the solvent used for dissolving the CTL-binders in which CTM's exhibit optimum charge transport properties. Limited is the range of solvents in which efficient CTM's are soluble. The range of solvents in which both CTL-binders and CTM's are soluble is extremely narrow and often limited to chlorohydrocarbons such as methylene chloride. Methylene chloride is an extremely powerful solvent and the range of CGL-binders which is totally insoluble in methylene chloride is extremely limited, unless the CGL-binder is crosslinked in a subsequent hardening process.
Hardening is considered here as a treatment which renders the binder of a charge generating layer of the photoconductive recording material insoluble in methylene chloride.